A fluid bearing such as a gas bearing or an oil bearing, and in particular a hydrodynamic bearing, tends to transmit a force to its casing which is proportional firstly to the stiffness of the bearing and secondly to its eccentricity, i.e. the distance between the inertia axis of the rotor and the axis of the bearing. Regardless of how well the rotor is balanced, the off-balance is never completely eliminated. The force transmitted to the casing thus tends to set up vibrations therein which in numerous cases are detrimental to the environment in which the rotary machine is located.
In order to remedy this drawback, proposals have already been made to compensate the forces exerted on the casing of the machine by a hydrodynamic bearing by using electrodynamic excitors which exert active damping on the casing, tending to reduce the action of the forces exerted thereon by the hydrodynamic bearing. Such a device which acts directly on the structure to be stabilized is not completely satisfactory since its action cannot avoid residual vibration, and above all it is not possible to make a device which is compact.
U.S. Pat. No. 4,626,754 describes a device for reducing the vibrations of rotary machines and applicable to the case where the rotor is mounted in the casing by means of an active magnetic suspension which includes at least one radial magnetic bearing. In this prior device, the vibration detectors disposed on the casing use the servocontrolled circuits of the active magnetic bearings to reduce the vibrations to which the casing is subjected. Such an active magnetic suspension of the rotor could thus be juxtaposed with a hydrodynamic bearing suspension in order to use the negative stiffness of the magnetic bearing to compensate for the disturbing forces created by the hydrodynamic bearings. However, this solution suffers from drawbacks due to the fact that it cannot prevent parasitic couples appearing on the stator due to the juxtaposition of axially offset magnetic and hydro-dynamic bearings.
The aim of the present invention is to remedy the above-mentioned drawbacks by providing a compact device capable of effectively reducing, and even of eliminating, the vibrations generated in the casing of the rotary machine by virtue of the machine having a fluid bearing for supporting its rotor.